Modern emerging data services and applications have put forward an ever-increasing bandwidth requirement for fiber-optic communication channels. To this end, we propose a novel symbol division multiplexing technology (SDM) by multiplexing/de-multiplexing of multiple quadrature amplitude modulation (QAM) symbols onto one complex constellation point. In our SDM scheme, every 7-bit 128QAM symbol is multiplexed per complex valued signal sequentially according to the optimal many-to-one mapping law, forming a 32QAM in the constellation and achieving an extra 40% gain for symbol capacity in an optical discrete multi-tone transmission system. The experiments prove that the SDM-32QAM successfully mitigates the signal impairments induced by fiber chromatic dispersion and Kerr nonlinearity, thus leading to 3.91-dB superior receiver power sensitivity and 2-dB enhancement of systematic tolerance to fiber nonlinear effect. The results highly motivate a fundamental paradigm in multiplexing techniques for optical fiber communication systems.